Amongst the many insect pests that are harmful to crops, Aleyrodidae (hereinafter whiteflies) represent a particular concern for growers, both in the UK and elsewhere. Indigenous and now established (formerly non-indigenous) whitefly species in the UK include Glasshouse whitefly (Trialeurodes vaporariorum/T. vap) and Bemisia afer. Current non-indigenous whitefly species of major concern include Tobacco whitefly (Bemisia tabaci/B. tabaci), Trialeurodes ricini and Trialeurodes abutiloneus. 
B. tabaci is a worldwide pest and virus vector. Originally known as a pest of sub-tropical crops, the species is now widely distributed under glass in temperate areas including most of Europe. It is not established in the UK but it could establish in protected environments, where it has the potential to be a major pest, particularly of glasshouse salad crops such as tomato and cucumber.
B. tabaci is a major vector of more than 110 virus species. In the UK, the risk of virus transmission presents a serious threat to protected crops, particularly vegetables. B. tabaci is a pest of an extremely wide range of host plants, and the number of recorded hosts is increasing. They include crops grown outside in the tropics and sub-tropics (including cotton, soyabean and cassava), vegetable and salad crops grown under glass in Europe (e.g. cucumber, aubergine, peppers and tomatoes) and ornamental plants (e.g. poinsettia).
Approximately 125 plant viruses, many of them causing potentially significant or devastating diseases, are transmitted by whiteflies, of which B. tabaci (approximately 110 viruses) is by far the most important vector. By contrast, Trialeurodes species transmit very few viruses and Bemisia afer only one.
B. tabaci and T. vap can be found on the upper and lower surfaces of leaves of crops grown in glasshouses. The adults feed on growing shoots and lay eggs that hatch into tiny white scales that remain attached to the underside of leaves. Direct damage to plants by B. tabaci and T. vap is caused by feeding activity and subsequent virus transmission. Indirect damage occurs due to contamination of leaves with honeydew, on which sooty mould develops and intercepts light, thereby reducing photosynthesis.
A major issue for growers can be damage to unripe crops which are exported where damage is not visible before ripening and not detectable until arrival at the target market.
Whitefly can be difficult to control because they have a very fast lifecycle. Any control method must be implemented early and regularly to control severe or early outbreaks.
Naturally, any efforts at controlling or containing whiteflies will be greatly enhanced if the particular whitefly species present can be identified. Indeed, the identification of indigenous and, potentially, non-indigenous whitefly species is an important precursor to the exclusion, containment, and eradication or control of these invertebrate pests and the viruses that they transmit.
A number of approaches to the identification and monitoring of whitefly species are known, with the trapping of whitefly being the most common. A well known whitefly trap (also suitable for use with other insect pests) is the Yellow Sticky Trap (YST), available from Agrisense-BCS Ltd. These traps comprise an adhesive applied to a plastic backing sheet. The backing sheet is coloured yellow so as to attract whiteflies. The backing sheet may be printed with a grid to assist in the counting of whiteflies. Typically the traps are suspended above the crop.
Such traps can also be used for monitoring and/or controlling other insects. For certain insects other than whiteflies, pheromones can be added to the trap to further increase the degree of attraction. An example of the application of an insect pheromone to a sticky trap is disclosed in US2006/0041018, wherein isobornyl-containing compounds are proposed as pheromones for monitoring and/or controlling Thysanoptera (thrips).
However, in the case of whiteflies, no attractant pheromone is available. Indeed no whitefly pheromones have been identified. In this connection, a compound that is a pheromone for one particular family of insects would not be expected to be a pheromone in respect of a different family, so it is not effective to use pheromones for other insects. This means that the control and monitoring of whiteflies presently relies on conventional trapping methods wherein colour (typically yellow) represents the only means of attraction.
In addition to traps, a physical barrier in the form of insect mesh (as available from Harold Horticulture) can reduce the number of whitefly attacks. A further approach is to plant a trap crop, which is more attractive to whiteflies than the crop itself. These can be planted between the crop (row intercropping) or around the crop (perimeter trap crop). An example of a trap crop is squash. A further related approach is to grow plants between the crop that naturally repel whiteflies, for example French Marigolds (Tagetes patula).
Biological control measures include the introduction of one or more natural predators selected from Encarsia formosa (parasitic wasp); Delphastus catalinae (parasitic ladybeetle); Eretmocerus eremicus (parasitic wasp—available for release in the UK under licence from DEFRA); Macrolophus caliginosus (predatory mind bug); Amblyseius swirskii (predator mite); Verticillium lecanii (a mycoinsecticide available as Mycotal from Plant Solutions Ltd); and Beauveria bassiana (a mycoinsecticide available as Naturalis-L from Troy Biosciences).
Chemical control measures include systemic insecticides such as imidacloprid, thiacloprid and azadirachtin. Azadiractin is available in commercial preparations Neemix™ and Bioneem™.
Given the considerable problem posed by whiteflies and the potential for harm if B. tabaci were to become established, there is a need for an improved approach to the monitoring and control of whiteflies.